diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index d3dbfbc44..81fb6b05a 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -155,7 +155,7 @@ void initNix()
        sshd). This breaks build users because they don't have access
        to the TMPDIR, in particular in ‘nix-store --serve’. */
 #if __APPLE__
-    if (getuid() == 0 && hasPrefix(getEnv("TMPDIR"), "/var/folders/"))
+    if (getuid() == 0 && hasPrefix(getEnv("TMPDIR").value_or("/tmp"), "/var/folders/"))
         unsetenv("TMPDIR");
 #endif
 }